Preparation of esters of aconitic acid



United States Patent 3,076,021 PREPARATTUN 9F ESTERS 0F ACONITICACTD Gaylord K. Finch and Charles D. Stringer, Kingsport, Tenn., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Dec. 5, 1958, Ser. No.

6 Claims. (Cl. 260-435) This invention concerns a method of obtaining aconitic acid esters particularly normaland isobutyl esters from the respective acetylated citric acid esters.

Aconitic acid esters have not been easy to obtain in good yield or at an economical price by previously known methods of preparation. As presently available, aconitic acid is a crude and expensive starting material for direct esterification. Dehydration of citric acid or its esters to aconitic acid or its esters has not been found to be practical inasmuch as considerable decomposition of the citric acid esters occurs at high temperatures which have been used in prior reactions.

The prior art methods teach methods of obtaining aconitic acid esters from alkaline earth aconitates. However, these methods have been found to be involved and expensive. The use of a vapor phase method of dehydrating citric acid esters has been difiicult to carry out since temperatures of 250 to 450 C. are not easily obtained using steam heat but require special heating apparatus such as electric, hot flue gas or heat exchange using hot oil.

We have discovered a process of obtaining esters of aconitic acid by the dehydration of acetylated citric acid esters in the liquid phase.

One object of this invention is to obtain aconitic acid esters by the liquid phase de-acetylation of citric acid esters at a reasonable temperature. Another object of this invention is to provide an economically feasible method of producing aconitic acid esters in good yield. A further object is to provide a process for obtaining aconitic acid esters which can be carried out in standard esterification equipment at temperatures obtainable using steam heat.

Our invention involves the low temperature pyrolysis of citric acid esters followed by molecular distillation. The following reaction showing the de-acetylation of acetyl tributyl citrate indicates the operation of our invention.

n-r'i-o o 0 can -0 0 0 mm ornco on (2-0 0 0 mm H l orno-o-c-Co o 0.119

' ethyl citrate, acetyl triisobutyl citrate, acetyl triamyl citrate, acetyl tri-2-ethylbutyl citrate, acetyl tri-Z-ethylhexyl citrate and many others.

The acetylated citric acid ester is charged into the reactor vessel and the pressure reduced to the approxi mate range of 1-50 millimeters of mercury. The charge is heated at 175-225 C. with or without agitation and with or without a catalyst, depending on the quality of the citrate ester being used and on the other reaction conditions, until the theoretical quantity of acetic acid is cracked from the molecule and collected in the distillation receiver.

Catalysts which may be used to speed the reaction in the practice of the invention are sodium hydroxide silica gel, activated alumina, iodine, silicon dioxide, silicaalumina, sodium acetate, etc. These can be used over the concentration range of 01-20% by weight of the 3,076,021 Fatented Jan. 29, 1963 "ice citrate, but we prefer to work in the range of 0.25.0%.

The rate of cracking is a function of time and temperature, and we have found aconitate esters to be quite stable to temperatures of ZOO-225 C. for periods 4-8 hours. The process may be run continuously by employing a series of reactors, one overflowing to the next until the desired degree of completion is achieved.

During the cracking operation the reaction product becomes dark brown in color and, in addition to the acetic acid collected, there is also obtained a small amount of an acetate ester corresponding to the alcohol radical of the citrate ester being cracked. This lower ester is formed through ester interchange. Decolorization of the dark crude product is neither beneficial nor neces sary since, on molecular distillation or on fractionation, a distillate of low color aconitate is obtained. Distillation of the aconitate is accomplished at reduced pressure and does not need to be preceded by neutralization or stripping. lt is not necessary to use an inert gas during cracking, which would require special condensers and scrubbers to separate out the acetic acid formed. A. refined aconitate ester of color 20-50 p.p.m. and acidity of 0.01 to 0.03% calculated as the dialkyl aconitate is obtained by simply distilling the crude reaction product.

The following examples are intended to illustrate our invention but not to limit it in any way.

Example 1 402 g. of commercial grade acetyl tributyl citrate was placed in a glass flask equipped with an agitator. The flask was attached to an 18 inch tall packed fractionating column with a vapor take-oil" head. The ester was heated six hours in the liquid phase over the temperature range of 196-203 C., while maintaining the system at 4-6 mm. pressure. During this time the acetic acid formed was condensed and its volume gave an indication of the extent or completion of the reaction. A conversion of 70.1% to tributyl aconitate was obtained, based on infrared analysis of the crude. The yield, based on acetyl tributyl citrate consumed, was 88%.

Example 2 402 g. of acetyl tributyl citrate was reacted as in Example 1, except with 1% sodium acetate present. In

3-4 hours heating in the liquid phase 90% of the theoretical acetic acid was cracked out. The temperature on this run was 197-199 C. The yield of molecularly distilled tributyl aconitate was 72%. Further fractionation of this product gave tributyl aconitate of 96.1% purity, n =1.4526, specific gravity at 28-1.020, color-50 APHA.

Example 3 One mole (570 g.) acetyl tri-Z-ethylhexyl citrate was pyrolyzed in the liquid phase as described in Example 2 for 5 hours at 200-215" C. The black crude product gave a distilled yield of 80% tri-octyl aconitate with a purity of Aconitic acid esters are valuable as plasticizers for synthetic resins and rubbers. For these applications a high quality product is desired which does not contain impurities which would be deleterious to the materials to which it is incorporated. Such a product is obtained using our process.

US. 2,445,911 shows the acetylation of citric acid esters with acetic .anhydride in the presence of sulfuric acid as a catalyst.

We claim:

1. A process for obtaining a trialkyl aconitate ester comprising heating an ester selected from the class consisting of acetyl triethyl citrate, acetyl tributyl citrate, acetyl triisobutyl citrate, acetyl triamyl citrate, acetyl triaoraoar U Z-ethylbutyl citrate, and acetyl tr'-2-ethylhexyl citrate at a temperature of about 183-225 C. in the liquid state at 1-50 mm. of mercury pressure in the presence of 01-10% by Weight of the citric acid ester of a catalyst selected from the class consisting of sodium hydroxide, iodine and sodium acetate.

2. A method of obtaining a trialkyl aconitate ester comprising heating an ester selected from the class consisting of acetyltriethyl citrate, .aeetyl tributyl citrate, acetyl triisobutyl citrate, acetyl triarnyl citrate, acetyl tri- Z-ethylbutyl citrate, and acetyl tri-2-ethylhexyl citrate by heating the citrate at 180-225 C. in the liquid state at 1-50 mm. of mercury pressure in the presence of 0.1- 10% by weight of the citric acid ester of sodium hydroxide. 3'A" method of obtainin a trialltyl aconitate ester comprising heating an ester selected from the class consisting of acetyl triethyl citrate, acetyl tributyl citrate, acetyl triisobutyl citrate, acetyl triamyi citrate, acetyl tri- Z-ethylbutyl citrate, and acetyi tri-Z-ethylhexyl citrate by heating the citrate at 186-225 C. in the liquid state at 1-50 mm. of mercury pressure in the presence of 0.1- 10% by weight of the citric acid ester of iodine.

4. A method of obtaining a trialkyl aconitate ester comprising heating an ester selected from the class consisting of acetyl triethyl citrate, acetyl tributyl citrate, acetyl triisobutyl citrate, acetyl triamyl citrate, acetyl tri- Z-ethylbutyl citrate and acetyl tri-Z-ethylhexyl citrate by heating the citrate at 180-225 C. in the liquid stage at 1-50 mm. of mercury pressure in the presence of 0.1- 10% by weight of the citric acid ester of sodium acetate.

5. A process for obtaining a triaikyl aconitate ester comprising heating an ester selected from the class consisting of acetyl triethyl citrate, acetyl tributyl citrate, acetyl triisobutyl citrate, acetyl triamyl citrate, acetyi tri- 2-ethylbutyl citrate and acetyl tri-Z-ethylhexyl citrate at a temperature of about 18-0-225 C. in the liquid stage at 1-50 mm. of mercury pressure in the presence of 0.1- 10% by Weight of acidic acid ester of a catalyst selected from the class consisting of sodium hydroxide, iodine and sodium acetate, and distilling the crude reaction product.

6. A process for obtaining a trialkyl aconitate ester comprising heating an ester selected from the class consisting of acetyl triethyl citrate, acetyl tributyl citrate, acetyl triisobutyl citrate, acetyl triamyl citrate, acetyl tri- Z-ethylbutyl citrate and acctyl' tri-Z-ethylhexyl citrate at a temperature of about 180-225 C. in the liquid stage at 1-50 of mercury pressure in the presence of 0.1- 10% by weight of a citric acid ester of a catalyst selected from the class consisting'of sodium hydroxide, iodine and sodium acetate, removing'ac'etic acid by vacuum distillation from the reaction continuously as formed, removing the reaction product and distilling the aconitate ester.

References Cited in the file of this patent UNITED STATES PATENTS Kirk OTHER REFERENCES Fisher et al.: Ind. Eng. Chem. 36, 229-234 (1944). Anschutz et al.: Ber. Deut. Chem. 18, 1953-1955 (1885).

2,375,563 May 8, 1945 

1. A PROCESS FOR OBTAINING A TRIALKYL ACONITATE ESTER COMPRISING HEATING AN ESTER SELECTED FROM THE CLASS CONSISTING OF ACETYL TRIETHYL CITRATE, ACETYL TRIBUTYL CITRATE, ACETYL TRIISOBUTYL CITRATE, ACETYL TRIAML CITRATE, ACETYL TRI2-ETHYLBUTYL CITRATE, AND ACETYL TRI-2-ETHYLHEXYL CITRATE AT A TEMPERATURE OF ABOUT 180-225*C. IN THE LIQUID STATE AT 1-50 MM. OF MERCURY PRESSURE IN THE PRESENCE OF 0.1-10% BY WEIGHT OF THE CITRIC ACID ESTER OF A CATALYST SELECTED FROM THE CLASS CONSISTING OF SODIUM HYDROXIDE, IODINE AND SODIUM ACETATE. 